Printed circuit boards are necessary as components for electronic apparatuses and electrical apparatuses. Printed circuit boards have various structures such as single-sided printed circuit boards, double-sided printed circuit boards, and multilayer printed circuit boards. Printed circuit boards are assembled by mounting devices such as resistors, capacitors, reactors, and transformers, by soldering.
Recently, to dissipate heat generated when power semiconductors or LED devices are operated, metal-base printed circuit boards including a copper foil layer, an insulating resin layer, and a metal base plate have been increasingly used. To conserve energy, power semiconductors that control electric power, and LEDs that can emit light with less electricity than incandescent light bulbs, have been increasingly employed.
For example, a metal-base printed circuit board has a structure in which an insulating resin layer having a thickness of approximately 100 μm to approximately 300 μm and a copper foil layer having a thickness of approximately 35 μm to approximately 140 μm are stacked in this order on a metal base plate having a thickness of approximately 1 mm to approximately 2 mm.
The insulating resin layer is obtained as a prepreg having a thickness of about 50 μm by applying a material obtained by adding an inorganic filler such as SiO2 or Al2O3 to an epoxy resin onto a copper foil layer, a PET film, or the like. One to several sheets of the prepreg are stacked on the metal base plate, and furthermore, the copper foil layer is stacked thereon. Then, these layers and plate are bonded together by hot pressing. A metal-base printed circuit board is obtained by etching the copper foil layer to any wiring pattern. As the metal-base printed circuit board, a printed circuit board described in Patent Document 1 is known, for example.
A point in a case in which a power semiconductor or the like is mounted on a metal-base printed circuit board is that the metal-base printed circuit board can dissipate heat (cool the power semiconductor or the like) appropriately during the operation thereof. As the power semiconductor, an insulated gate bipolar transistor (IGBT), a MOS-FET (metal oxide film semiconductor field-effect transistor), or the like is used. The electric power generated by the power semiconductor reaches several watts to several tens of watts. To dissipate the heat efficiently, it is desirable that the insulating resin layer have as high a thermal conductivity as possible.
Insulating resin layers generally used so far for metal-base printed circuit boards have thermal conductivities of about 1 W/m·K to 3 W/m·K. Recently, those having higher thermal conductivities have been demanded, and insulating resin layers having thermal conductivities improved to about 5 W/m·K to 6 W/m·K have been developed and used.
In the case of a metal-base printed circuit board with the thermal conductivity being about 1 W/m·K to 3 W/m·K, an inorganic filler such as SiO2 or Al2O3 is added at a ratio of about 80% by mass. To increase the thermal conductivity to about 5 W/m·K to 6 W/m·K, Al2O3 or the like is further added to about 80% by mass to 95% by mass, or an inorganic filler having a high thermal conductivity such as BN or AlN is added.
In general, the insulating resin layer used has a thickness of 80 μm to 200 μm. A voltage is to be applied between the copper foil layer and the metal base plate. The applied voltage varies depending on the purpose of the device, and for example, when a 1200 V IGBT element is used, a voltage of approximately 1000 V is applied to the insulating resin layer. Hence, the insulating resin layer has to provide reliable insulation for a long period.
In particular, it is necessary to prevent occurrence of electrochemical migration, which is one of the insulation degradation phenomena occurring in the wiring pattern of the copper foil layer. In this phenomenon, moisture absorption or dew condensation, which may occur during use of the device, causes decrease in insulation resistance in the wiring pattern of the copper foil layer, and copper ions are eluted from the copper foil and are deposited by reduction on the counter electrode, so that an electrically conductive path is formed, which leads to a short circuit.
The electrochemical migration may also occur in an insulating resin layer of a metal-base printed circuit board. Hence, it is necessary to prevent the electrochemical migration. In particular, copper ions are easily eluted in end portions of the copper foil layer and in the vicinities of the end portions where the electric field intensity is high.